Fuel cell power system and method of controlling a fuel cell power system

ABSTRACT

A fuel cell power system includes a fuel cell which has an optimal voltage; an energy storage device having a nominal voltage substantially similar to the optimal voltage of the fuel cell; and an electrical switch that, in operation, selectively electrically couples the fuel cell and the energy storage device to charge the energy storage device. A method includes providing a fuel cell having a nominal voltage; providing an energy storage device having a nominal voltage which is substantially similar to the nominal voltage of the fuel cell and electrically coupling the energy storage device to a load; and selectively electrically coupling the fuel cell to the energy storage device to substantially maintain the energy storage device above a predetermined voltage threshold.

RELATED PATENT DATA

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/916,791 and which was filed on Jul. 26, 2001, now U.S. Pat.No. 6,743,536, and which is further a continuation-in-part of U.S.patent application Ser. No. 09/577,407, filed on May 17, 2000, now U.S.Pat. No. 6,468,682, issued Oct. 22, 2002, both of which are incorporatedby reference herein.

TECHNICAL FIELD

The invention relates to an ion exchange membrane fuel cell and a methodof controlling an ion exchange membrane fuel cell. The invention alsorelates to an ion exchange membrane fuel cell power system incorporatinga fuel cell module and a method for improving performancecharacteristics of such a fuel cell power system. The invention alsorelates to methods and apparatus for supplying electrical energy to aload and compensating for variations in a load powered by a fuel cellsystem.

BACKGROUND OF THE INVENTION

Fuel cell systems are known in the art. A fuel cell is anelectrochemical device which reacts hydrogen and oxygen which is usuallysupplied from the air, to produce electricity and water. The basicprocess is highly efficient, and for those fuel cells fueled directly byhydrogen, pollution free. Further, since fuel cells can be assembledinto stacks of various sizes, power systems have been developed toproduce a wide range of electrical power outputs and thus can beemployed in numerous industrial applications. The teachings of prior artpatents, U.S. Pat. Nos. 6,030,718, and 6,096,449, are incorporated byreference herein.

A fuel cell produces an electromotive force by reacting fuel and oxygenat respective electrode interfaces which share a common electrolyte. Inthe case of a proton exchange membrane (PEM) type fuel cell, hydrogengas is introduced at a first electrode where it reacts electrochemicallyin the presence of a catalyst to produce electrons and protons. Theelectrons are circulated from the first electrode to a second electrodethrough an electrical circuit connected between the electrodes. Further,the protons pass through a membrane of solid, polymerized electrolyte (aproton exchange membrane or PEM) to the second electrode.Simultaneously, an oxidant, such as oxygen gas, (or air), is introducedto the second electrode where the oxidant reacts electrochemically inthe presence of the catalyst and is combined with the electrons from theelectrical circuit and the protons (having come across the protonexchange membrane) thus forming water and completing the electricalcircuit. The fuel-side electrode is designated the anode and theoxygen-side electrode is identified as the cathode. The externalelectric circuit conveys electrical current and can thus extractelectrical power from the cell. The overall PEM fuel cell reactionproduces electrical energy which is the sum of the separate half cellreactions occurring in the fuel cell less its internal losses.

Since a single PEM fuel cell produces a useful voltage of only about0.45 to about 0.7 volts D.C. under a load, practical PEM fuel cellplants have been built from multiple cells stacked together such thatthey are electrically connected in series. In order to reduce the numberof parts and to minimize costs, rigid supporting/conducting separatorplates often fabricated from graphite or special metals have beenutilized. This is often described as bipolar construction. Morespecifically, in these bipolar plates one side of the plate services theanode, and the other the cathode. Such an assembly of electrodes,membranes, and the bipolar plates are referred to as a stack. Practicalstacks have heretofore consisted of twenty or more cells in order toproduce the direct-current voltages necessary for efficient powerconversion.

The economic advantages of designs based on stacks which utilize bipolarplates are compelling. However, this design has various disadvantageswhich have detracted from its usefulness. For example, if theperformance of a single cell in a stack declines significantly or fails,the entire stack, which is held together in compression with tie bolts,must be taken out of service, disassembled, and repaired. In traditionalfuel cell stack designs, the fuel and oxidant are directed by internalmanifolds to the electrodes. Cooling for the stack is provided either bythe reactants, natural convection radiation, and possibly supplementalcooling channels and/or cooling plates. Also included in the prior artstack designs are current collectors, cell-to-cell seals, insulation,piping, and various instrumentation for use in monitoring cellperformance. The fuel cell stack, housing, and associated hardware makeup the operational fuel cell plant. Such prior art designs are undulylarge, cumbersome, and quite heavy. Any commercially useful PEM fuelcell designed in accordance with the prior art could not be manipulatedby hand because of these characteristics.

Fuel cells are, as a general matter, relatively slow to respond toincreased load demands. When a fuel cell is used in a power distributionsystem, loads may vary over time. At some times, there may be spikes inthe load. Because a certain amount of time is normally required to startup a fuel cell, additional fuel cells or fuel cell subsystems cannot beinstantaneously brought on-line to handle instantaneous spikes in theload. At the same time, a spike in the load that exceeds the capacity ofan on-line fuel cell can potentially damage the fuel cell. Thus, fuelcell overcapacity may be provided in prior art systems in order tohandle short temporary spikes in demand. This type of design isinefficient and wasteful.

Fuel cells have, from time to time, been used in conjunction with chargestorage devices, such as batteries, which can provide a moreinstantaneous power supply for given application needs. In mostinstances, the direct current (DC) power which a fuel cell power systemproduces must be converted to alternating current (AC) for mostapplications. In this regard, an inverter is normally used to convertthe fuel cells DC power to AC. As a general matter, inverters generallyfunction within a specified DC input voltage range. In some previousapplications, the fuel cell and charge storage device have been coupledto an inverter which functions at the optimal voltage of either the fuelcell or the charge storage devices. In this arrangement, the voltage ofthe fuel cell was raised or lowered as appropriate, to provide optimumfunctioning of the system. Still further, altering the voltage resultedin decreased efficiency by way of heat loss incumbent in the conversionprocess.

The present invention addresses many of the shortcomings attendant withthe prior art practices. For example, previous prior art applicationswhich provide both a fuel cell and a charge storage device in thearrangement discussed above, have been unduly complex and haveexperienced as noted above, decreased efficiency by way of heat lossescaused by the lowering of the voltages generated by the fuel cell tomake the fuel cell voltage match, as closely as possible, the voltagecapacity of the charge storage devices used with same.

Further, designers have long sought after means by which current densityin self-humidified PEM fuel cells can be enhanced while simultaneouslynot increasing the balance of plant requirements for these same devices.

Accordingly, an improved ion exchange membrane fuel cell,is described incombination with a method for controlling same which addresses theperceived shortcomings associated with the prior art designs andpractices while avoiding the shortcomings individually associatedtherewith.

Attention is directed toward the following patents, which areincorporated herein by reference: U.S. Pat. No. 6,028,414 to Chouinardet al.; U.S. Pat. No. 5,916,699 to Thomas et al.; and U.S. Pat. No.5,401,589 to Palmer et al. U.S. Pat. No. 5,401,589 to Palmer et al.discloses a rechargeable battery provided in parallel with a fuel cellelectrical output together with appropriate charging, switching andcontrol means so that a sudden increase in power demand can be met byboth the fuel cell and battery working together and/or a sudden decreasein power demand may be met by charging of the battery.

U.S. Pat. No. 5,916,699 to Thomas et al. discloses an energy storagesystem including a first energy storage device, such as a secondary orrechargeable battery, and a second energy storage device, such as acapacitor, fuel cell or flywheel. The second energy storage deviceprovides intermittent energy bursts to satisfy the power requirementsof, for example, pulsed power communication devices.

U.S. Pat. No. 6,028,414 to Chouinard et al. discloses a fuel cellstand-by energy supply system incorporating storage battery(ies) forsupplying electrical power, the battery(ies) being recharged by the fuelcell.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a perspective, side elevation view of an ion exchange membranefuel cell module of a fuel cell power system embodying the presentinvention.

FIG. 2 is a perspective, exploded, side elevation view of an ionexchange membrane fuel cell module.

FIG. 3 is a perspective, partial, exploded, side elevation view of anion exchange membrane fuel cell module.

FIG. 4 is a fragmentary, perspective, greatly enlarged, exploded view ofa membrane electrode diffusion assembly employed with the ion exchangemembrane fuel cell module.

FIG. 5 is a fragmentary, side elevational view of a fuel distributionassembly utilized with the ion exchange membrane fuel cell module.

FIG. 6 is a second, fragmentary, side elevational view of the fueldistribution assembly taken from a position opposite to that seen inFIG. 5.

FIG. 7 is a second, perspective, partial, exploded view of a portion ofthe ion exchange membrane fuel cell module of the present invention.

FIG. 8 is a perspective view of an ion exchange membrane fuel cellsubrack and associated fuel gas supply.

FIG. 9 is a fragmentary, transverse, vertical sectional view taken froma position along line 8-9 of FIG. 8.

FIG. 10 is a fragmentary, schematic representation of an ion exchangemembrane fuel cell module, and associated power systems.

FIG. 11 is a block diagram illustrating a plurality of fuel cellsubracks or sub-systems of the type shown in FIG. 8 and respectivelyselectively coupled to an energy storage device via circuitry such as isshown in FIG. 12.

FIG. 12 is a schematic representation of an exemplary configuration ofpower conditioning circuitry.

FIGS. 13A, 13B and 13C together define a flowchart illustrating logicperformed by a controller that controls the power conditioning circuitryassociated with each subrack or sub-system to selectively couple eachsubrack or sub-system to the energy storage device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

One aspect of the invention provides a fuel cell power system comprisinga fuel cell which has an optimal voltage; an energy storage devicehaving a nominal voltage substantially similar to the optimal voltage ofthe fuel cell; and an electrical switch that, in operation, selectivelyelectrically couples the fuel cell and the energy storage device tocharge the energy storage device.

Another aspect of the invention is to provide an energy storage device,such as an ultra-capacitor or battery, coupled to a load. The batteryand ultra-capacitor are useful, for example, for absorbing spikes orother changes in the load. The battery and ultra-capacitor are suppliedwith electricity generated by a fuel cell which is made up of subracksor individual fuel cell sub-systems. Circuitry is provided whichmeasures or monitors the voltage of the battery and/or theultra-capacitor and selectively couples individual fuel cell subracks orsub-systems to the battery and/or ultra-capacitor in response to themeasured or monitored voltage of the battery.

Another aspect of the present invention relates to a fuel cell powersystem comprising a fuel cell which, in operation, converts chemicalenergy into direct current electrical energy, the fuel cell beingdefined by a plurality of independently operable fuel cell sub-systems;a DC bus; a switching circuit electrically coupled with the fuel cellsub-systems and configured to independently selectively couple the fuelcell sub-systems to the DC bus; and an energy storage device such as abattery and/or ultra-capacitor electrically coupled with the DC bus andconfigured to be coupled to a load, and wherein the switching circuitselectively electrically couples a selectable number of the fuel cellsub-systems to the DC bus to supply direct current electrical energy tothe energy storage device to charge the energy storage device.

Yet another aspect of the invention relates to a fuel cell power systemcomprising a plurality of fuel cells, having substantially similarnominal voltages; an energy storage device such as a battery and/orultra-capacitor having a nominal voltage substantially similar to thatof each of the fuel cells; and electrical switching circuitryelectrically coupled to the fuel cells and the energy storage device,and which is configured to electrically couple a selectable number ofthe fuel cells to the energy storage device to maintain the voltage ofthe energy storage device above a predetermined voltage.

Still another aspect of the invention relates to a fuel cell powersystem comprising a fuel cell which has a nominal operating voltage; anenergy storage device having a nominal voltage substantially similar tothe nominal operating voltage of the fuel cell; an electrical switchselectively coupling the fuel cell to the energy storage device; and acontroller coupled in voltage sensing relation relative to the fuelcell, and the energy storage device, and further coupled in controllingrelation relative to the electrical switch, the controller selectivelycontrolling the electrical switch to selectively electrically couple thefuel cell to the energy storage device to maintain the voltage of theenergy storage device above a predetermined threshold.

Yet still another aspect of the present invention relates to a fuel cellpower system comprising a power conditioning device having a DC input,and having an electrical output, and which is configured to be coupledto a load; an energy storage device such as a battery and/orultra-capacitor coupled to the DC input; a plurality of fuel cellsub-systems; and electrical circuitry for measuring the voltage of theenergy storage device and selectively couple a selectable number of thefuel cell sub-systems to the energy storage device in response to themeasured voltage of the energy storage device.

Still another aspect of the present invention relates to a methodcomprising: (a) measuring the voltage of the energy storage device; (b)determining if the measured voltage is less than a first threshold and,if so, proceeding to step (c) and, if not, proceeding to step (d); (c)de-coupling all the sub-systems from the energy storage device; (d)determining if the measured voltage is greater than or equal to a secondthreshold and, if so, proceeding to step (e) and, if not, proceeding tostep (g); (e) determining if all sub-systems are de-coupled from theenergy storage device and, if so, proceeding to step (a) and, if not,proceeding to step (f); (f) decoupling all of the sub-systems from theenergy storage device; (g) determining if the measured voltage isgreater than or equal to a third threshold and, if so, proceeding tostep (h) and, if not, proceeding to step (j); (h) determining if allsub-systems are de-coupled from the energy storage device and, if so,proceeding to step (a) and, if not, proceeding to step (j); (i)decoupling one of the sub-systems coupled to the energy storage devicefrom the energy storage device (j) determining if the measured voltageis greater than or equal to a fourth threshold and, if so, proceeding tostep (k) and, if not, proceeding to step (m); (k) determining if allsub-systems are coupled to the energy storage device and, if so,proceeding to step (a) and, if not, proceeding to step (l); (l) couplingone of the sub-systems de-coupled from the energy storage device to theenergy storage device; (m) determining if all sub-systems are coupled tothe energy storage device and, if so, proceeding to step (a) and, ifnot, proceeding to step (n); and (n) coupling all sub-systems to theenergy storage device.

A further aspect of the present invention relates to a method comprisingproviding a fuel cell having a nominal voltage; providing an energystorage device having a nominal voltage which is substantially similarto the nominal voltage of the fuel cell and electrically coupling theenergy storage device to a load; and selectively electrically couplingthe fuel cell to the energy storage device to substantially maintain theenergy storage device above a predetermined voltage threshold.

Another aspect of the present invention relates to a method comprisingproviding a plurality of independently operable fuel cells which convertchemical energy into direct current electrical energy; providing anenergy storage device; coupling the energy storage device to a load;monitoring the voltage of the energy storage device; and varying thenumber of the fuel cells coupled to the energy storage device based uponthe voltage of the energy storage device.

The ion exchange membrane fuel cell power system 5 (FIG. 11) of thepresent invention is made up of a plurality of fuel cell modules 10,only one of which is shown in FIG. 1. As seen in FIG. 11 the ionexchange membrane fuel cell power system 5 comprises a plurality ofsubsystems 210. Each subsystem or subrack 210 includes a given number ofhand-manipulatable modules 10 (FIG. 1). The modules 10 have a main body11 which has a forward edge 12; an opposite, rearward edge 13; top andbottom surfaces or edges 14 and 15; and opposite sidewalls generallyindicated by the numeral 16. Each facet of the main body of the module11 will be discussed in greater detail hereinafter. Yet further itshould be understood that the present invention could be employed withconventional stack-like technology wherein the individual subsystemscomprise fuel cell stacks arranged in a manner which is consistent withthe further teachings of this application. Moreover, the presentinvention works particularly well with the fuel cell construction foundin U.S. Pat. No. 6,030,718, the teachings of which are incorporated byreference herein.

As best seen in FIGS. 2 and 3, the main body of the module 11 includes anonconductive, dielectric support member generally indicated by thenumeral 20. The support member can be fashioned out of various syntheticpolymeric substrates. The support member has (see FIG. 3) a main body21, which is defined by a forward peripheral edge 22; a rearwardperipheral edge 23; a top peripheral edge 24; an opposite, bottomperipheral edge 25; and opposite sidewalls generally indicated by thenumeral 26.

As best seen in FIG. 2, a pair of recessed channels 30 are formed in theforward peripheral edge 22. Further, a plurality of fastener receivingpassageways or apertures 31 are also formed in the forward peripheraledge 22. Yet further, and as seen in FIG. 3, a plurality of spaced ribs32 are borne by, or made integral with the respective sidewalls 26 andare disposed in spaced relation, one to the other. Fastener passagewaysor apertures 33 are formed through each of the ribs. Further, cavities34 are defined between the respective ribs 32 on each sidewall. Thecavities 34 formed on each of the sidewalls are disposed insubstantially opposed relation one to the other. This is seen in FIG. 3.

Further, as best seen in FIG. 3, orientation members 35 are disposedbetween each of the ribs 32 and define a space therebetween. A pair ofmounting tabs 36 are provided in spaced relationship, one to the other,on the rearward peripheral edge 23 of the main body 21. A pair ofsubstantially coaxially aligned apertures 37 are individually formed ineach of the mounting tabs 36 and are operable to receive a fastenertherethrough.

A fuel coupling 40 is made integral with or forms a portion of therearward peripheral edge 23 of the support member 20. The fuel coupling40 includes a fuel delivery passageway 41 which is substantially Tshaped and which is defined by an intake end 42 and a pair of exhaustends labeled 43. Additionally, the fuel coupling also includes anexhaust passageway 44 which is also substantially T shaped and which isdefined by a pair of intake ends 45, and an exhaust end 46. Theoperation of the fuel coupling 40 will be discussed in greater detailhereinafter.

As best seen in FIGS. 2 and 3, individual conductor plates which aregenerally designated by the numeral 50 are matingly received within theindividual cavities 34 which are defined by the support member 20. Theconductor plates which are fabricated from an electrically conductivesubstrate, have a substantially planar main body 51, which has a firstend 52, and an opposite, second end 53. The main body 51 further has aconductive tab 54 which extends outwardly relative to the first end 52,and which is oriented between the individual orientation members 35. Theconductive tab extends substantially normally outwardly relative to thetop peripheral edge 24 of the support member 20. As will be recognized,the main body 51 matingly rests between the individual ribs 32 whichdefine, in part, the respective cavities 34.

As best seen in the exploded view of FIG. 3, a cathode current collectoris generally designated by the numeral 60, and rests in ohmic electricalcontact with the main body 51 of the individual conductor plates 50. Thecathode current collector, which is fabricated from an electricallyconductive substrate, has a main body 61 which has opposite first andsecond ends 62 and 63, respectively. The cathode current collectorsimultaneously performs the functions of current collection, forceapplication and heat dissipation. Still further, the main body 61 of thecurrent collector 60 is defined by a peripheral edge 64.

As best seen in the exploded view of FIGS. 4 and 7, the ion exchangemembrane fuel cell module 10 includes a plurality of membrane electrodediffusion assemblies generally indicated by the numeral 100. Each of themembrane electrode diffusion assemblies have an anode side 101, and anopposite cathode side 102. Still further, each of the membrane electrodediffusion assemblies is defined by a peripheral edge 103, and furtherhas formed in its anode side, a plurality of interlinking channels 104.The membrane electrode diffusion assembly 100, as noted above, is formedof a solid ion conducting membrane 105 which is sealably mounted orreceived in each of the respective cavities 34. In this arrangement, thecathode side 102 of each membrane electrode diffusion assembly 100 isheld in spaced relation relative to the support member 20 by deformableelectrically conductive members 70 (FIGS. 2 and 3) of the cathodecurrent collector 60. This spacial arrangement, which is provided by thecathode current collector, facilitates, in part, heat dissipation fromthe module 11. As described, above, the membrane electrode diffusionassembly 100; associated cathode current collector 60; and supportmember 20, in combination, define a cathode air passageway 106therebetween (FIG. 10). The construction of a suitable membraneelectrode diffusion assembly was described in our earlier U.S. Pat. No.6,030,718. This earlier patent is incorporated by reference herein, andfurther discussion regarding the construction of the membrane electrodediffusion assembly is not undertaken herein.

As will be appreciated, from a study of FIG. 10, the cathode airpassageway 106 is defined or otherwise oriented on each side 26 of thesupport member 20. Therefore, the main body of the module 11 has abifurcated cathode air flow. As will be appreciated, while the earlierdescribed membrane electrode diffusion assembly was directed to a protonexchange membrane, the fuel cell power system 10 of the presentinvention is not limited solely to a type having proton exchangemembranes, but also may utilize anion exchange membranes.

As best seen by reference to FIGS. 5, 6 and 7, a fuel distributionassembly, which is generally indicated by the numeral 110, is coupled influid flowing relation relative to the anode side 101 of each of themembrane electrode diffusion assemblies 100. Each fuel distributionassembly 110 is coupled with a source of a fuel 341 and/or 342 (FIG. 8)which may be substantially pure, or which is diluted to various degrees.Such may be achieved if the fuel cell power system 5 was coupled with areformer which would produce a stream of hydrogen from a source ofhydrocarbon such as gasoline, natural gas, propane, etc. If the fuelcell power system 10 was fabricated in the nature of a proton exchangemembrane fuel cell, the dilute fuel supply would include hydrogen. Theconcentration of the hydrogen in the dilute fuel would normally be in arange of about 30% to about 80% by volume.

When supplied with this dilute fuel mixture (regardless of the type),the main body of the fuel cell modules 11 produce an average currentdensity of at least about 350 mA per square centimeter of surface areaof each anode side 101 at a nominal voltage of 0.5 volts. Further, theinterlinking channels 104 formed in the surface of the anode side 101facilitate the distribution of the dilute fuel substantially about theentire surface area of the anode side 101. In this arrangement, ifcontaminants are introduced by way of the dilute fuel mixture or otherblockage occurs, the interlinking channels 104 provide a convenientpassage by which the fuel may reach substantially the entire surfacearea of the anode side 101, even though some portions of theinterlinking channels 104 may be blocked or otherwise substantiallyoccluded. As noted above, the dilute fuel may be supplied by a reactor342 (FIG. 8) which receives a hydrocarbon based fuel, and then through achemical reaction fractionates the hydrocarbon source to liberate adilute stream of hydrogen which is mixed with other substances. In thealternative, the fuel may be supplied by a pressurized container 341.These alternative arrangements are shown in FIG. 8.

As best seen in FIGS. 5 and 6, each of the fuel distribution assemblies110 include a main body 111 which has an inside facing surface 112,(FIG. 6) and an outside facing surface 113 (FIG. 5). The main body 111further defines an intake plenum 114, and an exhaust plenum 115.Further, a fluid coupling 116 (FIG. 1) is mounted in fluid flowingrelation relative to the individual intake and exhaust plenums 114 and115 respectively. A reduced dimension orifice 114 a (FIG. 5) is formedin the main body and communicates with the intake plenum. This reduceddiameter orifice operates to create a pressure differential in therespective apertures or cavities 120 during certain operationalconditions to facilitate the clearance of contaminants or otherobstructions which may be blocking any of the channels 104 which areformed in the membrane electrode diffusion assembly 100. A plurality ofcavities or apertures 120 are formed in the main body 111, and extendbetween the inside and outside facing surfaces 112 and 113,respectively. The cavities or apertures 120 are disposed in spacedrelation, one to the other, and when assembled, the cavities 120 receivethe individual membrane electrode diffusion assemblies 100. As best seenin FIG. 5, a plurality of channels or passageways 121 are formed in themain body 111, and couple the individual cavities 120 in fluid flowingrelation with the respective intake and exhaust plenums 114 and 115.Additionally, a plurality of fastener apertures 109 are formed in themain body. As further seen in FIG. 7, a sealing member 122 lies incovering relation relative to the individual channels 121.

As best seen in FIG. 1, a plurality of conduits 150 couple in fluidflowing relation the fuel coupling 40 with the fuel distributionassembly 110. Two of the conduits designated as 151 allow a dilute fuelmixture to be delivered by way of the intake plenum 114 to theindividual membrane electrode diffusion assemblies 100. Thereafter, anyremaining fuel, and associated by-products of the chemical reaction arereceived back into the exhaust plenum 115 and then flow by way ofconduits 152 to the fuel coupling 40 and then to the exhaust passageway44.

First and second pressure sensitive adhesive seals 123 and 124,respectively are provided, and are disposed in juxtaposed relationrelative to the opposite inside and outside facing surfaces 112 and 113,respectively. Each of the seals 123 and 124 have apertures 125 formedtherein which are substantially coaxially oriented relative to therespective cavities 120. As will be recognized, the cavities 120 whichare formed in the main body 111 of the fuel distribution assembly 110,matingly cooperate and are substantially coaxially aligned with theindividual cavities 34 which are formed in the nonconductive supportplate 20. As will be recognized, and following the assembly of same, therespective membrane electrode diffusion assemblies 100 are individuallyreceived in mating relation in each of the cavities 120 and 34 which aredefined by both the fuel distribution assembly 110, and the supportmember 20. Further, a plurality of fastener apertures 126 are formed inthe individual seals 123, and 124, and are operable to receive fastenerswhich will be discussed in greater detail hereinafter.

Lying in immediate juxtaposed relation relative to the second pressuresensitive adhesive seal 124 is an anode current collector which isgenerally designated by the numeral 140. Additionally, and as seen inFIG. 7, a substantially rigid sealing plate 130 is provided and which isjuxtaposed relative to the cathode side 102 of the membrane electrodediffusion assembly 100. The sealing plate 130 has a main body 131 whichdefines a plurality of apertures 132 which matingly receive, in part,the respective membrane electrode diffusion assemblies 100. Stillfurther, the main body has a plurality of fastener apertures 133 formedtherein and which when assembled, are substantially coaxially alignedwith the aforementioned fastener apertures formed in the earlierdescribed portions of the fuel cell module 11.

Each anode current collector 140 lies in ohmic electrical contactagainst the anode side 101 of each of the membrane electrode diffusionassemblies 100 and further is oriented in heat receiving relationrelative thereto. The anode current collector 140 has an electricallyconductive main body 141, which has an inside facing surface 142, whichlies against the anode side 101 of the membrane electrode diffusionassembly 100, and an opposite outside facing surface 143. Still further,a plurality of fastener apertures 144 are formed in the main body 131and are operable to be substantially coaxially aligned relative to theother fastener apertures 126 formed in the various seals 123,124, and inthe fuel distribution assembly 110.

As seen in FIG. 7, an electrically insulative member or gasket 160 ismounted or oriented in juxtaposed relation relative to the outsidefacing surface 143 of the anode current collector 140. This insulativemember has a main body 161 which has an inside facing surface 162 whichrests in contact with the outside facing surface 143 of the anodecurrent collector, and further has an outside facing surface 163.Further, a plurality of fastener apertures 164 are operable to becoaxially aligned with the previously described fastener aperturesformed in the remaining parts of the ion exchange membrane fuel cellpower system 5.

As best seen in FIG. 7, an anode heat sink 170 is oriented in juxtaposedrelation relative to the insulative member 160, and further, is mountedin heat receiving relation relative to the anode sides 101 of each ofthe membrane electrode diffusion assemblies 100 to conduct heat energygenerated by the ion exchange membrane module 11 away from the membraneelectrode diffusion assembly 100. In this arrangement, the fueldistribution assembly 110 is located substantially between the anodeside 101 of the membrane electrode diffusion assembly 100, and the anodecurrent collector 140. The anode heat sink 170 has a main body 171 whichhas an inside facing surface 172, which lies in juxtaposed relationrelative to the insulative member 160, and an opposite outside facingsurface 173. Similarly, and as discussed above, numerous fastenerapertures 174 are formed therein, and which are substantially coaxiallyaligned with the remaining fastener apertures which are formed in theearlier disclosed portions of the ion exchange membrane fuel cell module10. Fasteners 175 are provided and are received in these coaxiallyaligned fastener apertures such that the module is held firmly together.These fasteners 175 along with the respective current collectors 60create pressure sufficient to allow the individual current collectors 60and 140 to make effective ohmic electrical contact with the anode andcathode sides 101 and 102 respectively of the membrane electrodediffusion assembly 100. As will be recognized from the discussion above,the anode current collector 140 is substantially electrically isolatedfrom the anode heat sink 170. Additionally, the anode heat sink hassufficient thermal conductivity such that it substantially inhibits theformation of a temperature gradient across the membrane electrodediffusion assembly 100 during operation of the ion exchange membranefuel cell module 10.

A handle assembly is generally indicated by the numeral 190 and is bestseen in FIG. 2. As shown therein, the handle assembly 190 has a backplate generally indicated by the numeral 191, and which is defined by afront surface 192, and an opposite rear surface 193. Formed through thefront and rear surfaces is an aperture 194 which matingly receives themember 84 which is mounted on the main body 81 of the current conductorassembly 80. Still further, a pair of handles 195 are fastened on thefront surface 192, and additionally, a plurality of fastening apertures196 are formed through the front and rear surfaces 192 and 193 and areoperable to receive fasteners 197 which threadably engage the fastenerapertures 31, which are formed in the forward edge 23 of the supportmember 20. The handles permit the module 10 to be easily manipulated byhand, and removed without the use of any tools, when utilized with asubrack or sub-system which will be discussed in greater detailhereinafter.

The ion exchange membrane fuel cell power system 5 includes a pluralityof subracks or sub-systems 210, only one of which is shown in FIGS. 8and 9, and which is generally indicated by the numeral 210. Each subrack210 releasably supports a plurality of ion exchange membrane fuel cellmodules 10 in an operable arrangement. Each subrack 210 includes aprincipal enclosure 211. The principal enclosure is defined by a topsurface 212; bottom surface 213; front sidewall 214; rear sidewall 215;left sidewall 216, and right sidewall 217. The respective sidewalls 212through 217 define an internal cavity 220 (FIG. 9). In this arrangement,the principal enclosure will receive multiple fuel cell modules 10, eachenclosing a membrane electrode diffusion assembly 100.

As seen in FIG. 8, the ion exchange membrane fuel cell power system 5 isconfigured in a manner where at least one of the fuel cell modules 10can be easily removed from at least one of the subracks 210 by hand,while the remaining modules continue to operate. As noted above thisremoval is normally accomplished without the use of any tools, howeverit may be necessary in some commercial or industrial applications wherevibration, and other outside physical forces may be imparted to thesystem, to use threaded fasteners and the like to releasably secure theindividual modules to the subrack 210 to prevent the unintentionaldisplacement or dislocation of the respective modules from the subrack210. If utilized, the hand tools which will be employed will be simplehand tools, and the removal will be accomplished in minutes, as opposedthe prior art stack arrangements where replacement of a damaged membraneelectrode assembly (MEA) may take hours to accomplish. It should beunderstood that the terms “subrack” and “sub-system” as used in thefollowing claims do not necessarily imply that a rack or shelf isrequired, only that the sub-system, or a portion thereof, is operableindependently whether or not other sub-system, or a portion thereof, ofthe fuel cell power system 5 are functioning.

As best seen by reference to FIG. 9, an aperture 230 is formed in thetop surface 12 of the subrack 210, and further, the cavity 220 iscomprised of a first or fuel, cell module cavity 231, and a secondcavity or electrical control bay 232. As best seen by reference to FIG.8, a plurality of individual module apertures 233 are formed in thefront surface 214 of the principal housing 211, and are operable toindividually receive the respective fuel cell modules 10, and positionthem in predetermined spaced relation, one to the other.

The fuel cell module cavity 231 is further defined by a supportingmember or shelf 234 (FIG. 9) which orients the individual fuel cellmodules 10 in a predetermined substantially upright orientation withinthe cavity 231. Additionally, the fuel cell module cavity 231 is definedby a rear wall 235 which supports a DC bus 236 in an orientation whichwill allow it to releasably, matingly, electrically couple with thecurrent conductor assembly 80 (FIG. 2) which is borne by the fuel cellmodule 10. Yet further, and as seen in the cross sectional view of FIG.9, the rear wall 235 further supports a fuel supply line 237 and abyproduct removal line 238. These are operable to be releasably coupledin fluid flowing relation with respect to the fuel delivery passageway41 and the exhaust passageway 44 of the fuel coupling 40.

As best seen in FIG. 9, the second cavity or electrical control bay 232encloses a digital or analog controller 250 which is electricallycoupled with the respective ion exchange membrane fuel cell modules 10,and a power conditioning assembly 260 which is electrically coupled withthe DC bus 236, and the controller 250, and which is operable to receivethe electrical power produced by the ion exchange membrane fuel cellmodules 10. The operation of the controller 250 and power conditioningassembly 260 and related control circuitry is discussed in our priorU.S. application Ser. Nos. 09/108,667 (now U.S. Pat. No. RE39,556) and09/322,666 (now U.S. Pat. No. 6,387,556), which are incorporated byreference herein, except that operation of the controller 250 as itrelates to opening and closing subracks 210 is discussed below ingreater detail.

As further seen in FIG. 9, an aperture 270 is formed in the rear wall215 of the principal enclosure 211, and is operable to receive an airfilter 271 which is operable to remove particulate matter from anoutside ambient air stream passing therethrough and into the principalenclosure 211.

As best seen by the cross sectional view in FIG. 9, the subrack 210includes an air distribution plenum 290 which is coupled in fluidflowing relation relative to each of the ion exchange membrane fuel cellmodules 10. The air distribution plenum 290 has a first or intake end291 which receives both air which has previously come into contact witheach of the ion exchange fuel cell modules 10, and air which comes fromoutside of the respective ion exchange membrane fuel cell modules.Further, the air distribution plenum has a second or exhaust end 292which delivers an air stream to each of the ion exchange fuel cellmodules 10. Disposed intermediate the first or intake end 291, and thesecond or exhaust end 292 is an air mixing valve 293 which is coupled tothe air distribution plenum 290, and which meters the amount of airwhich is passed through the respective ion exchange membrane fuel cellmodules 10 and is recirculated back to the ion exchange fuel cellmembrane modules and by way of the air filter 271. As illustrated, themixing valve 293 selectively occludes an aperture 294 which is formed inthe rear wall 215 of the subrack 210.

An air movement assembly such as a fan 295 is provided, and is mountedalong the air distribution plenum 290. As shown in FIG. 9, the airmovement assembly 295 is positioned near the intake end 291, and issubstantially coaxially aligned with the aperture 230 which is formed inthe top surface 212 of the subrack 210. The air mixing valve and the fanassembly 293 and 295 respectively are electrically coupled with thecontroller 250 and are controlled thereby. The air mixing valve 293comprises a pivotally movable valve member 296 which can be moved from afirst occluding position 297 relative to the aperture 294, and a second,substantially non-occluding position 298 as shown in phantom lines.

As will be recognized, when the valve member 296 is in the secondnon-occluding position, air received in the intake end 291 and which haspreviously passed through the individual fuel cell modules will pass outof the principal enclosure 211 and then be exhausted to the ambientenvironment. On the other hand, when the valve member 296 is in theoccluding position 297 air from the intake end 291 which has passedthrough the fuel cell module 10 will return to the exhaust end and thenpass through the modules 10 and return again to the intake end. As willbe recognized, by controlling the relative position of the valve member296, temperature as well as relative humidity of air stream 299 can beeasily controlled. Still further, in the occluding position 297, airfrom ambient will continue to enter the air distribution plenum by wayof the air filter 270.

More specifically, the, air stream 299 which is supplied to the fuelcell modules is provided in an amount of at least about 5 to about 1000times the volume required to support a fuel cell chemical relation whichproduces water vapor as a byproduct. The present air plenum arrangementprovides a convenient way by which the air stream delivered to thecathode side 102 can be humidified by the water vapor generated as abyproduct of the chemical reaction taking place on the cathode.Additionally, during cold operating conditions, this same air, which hasnow been heated by each of the fuel cell modules 10, will contribute inbringing the entire fuel cell up to normal operating temperatures.Further, the air mixing valve 293 limits the amount of air which haspreviously passed through the modules 10 and which is added to the airdistribution plenum 290. This resulting recirculated air stream andfresh ambient air forms an air stream having substantially optimaloperating characteristics which maximizes the current densities andoutputs of the respective membrane electrode diffusion assembliesenclosed within each of the fuel cell modules 10.

Referring now to FIG. 10, what is shown is a greatly simplified,exaggerated, partial, and cross-sectional view of an ion exchangemembrane fuel cell module 10 which is positioned in an operationalrelationship relative to the air distribution plenum 290. Thisparticular sectional view, which does not include many of thesubassemblies previously discussed, is provided to illustrate theprincipals that will be set forth below. As seen in FIGS. 9 and 10, andas discussed above, the subrack 210 includes an air distribution plenum290 which provides a stream of air 299 to each of the ion exchange fuelcell modules 10 which are received in an operational position on theshelf or supporting member 234. The air stream 299 exits from theexhaust end 292 and then becomes a bifurcated air flow which isgenerally indicated by the numeral 320. The bifurcated air flow 322comprises a first cathode air stream 321, which is received in therespective ion exchange membrane fuel cell modules 10; and a secondanode heat sink air stream which is generally indicated by the numeral322. As will be recognized by a study of FIG. 10, the first cathode airstream 321 enters the ion exchange membrane fuel cell module 10, and isfurther bifurcated into a first component 323 which moves along one ofthe cathode air passageways 106 which is defined on one side of thesupport member 20. Further, the first cathode air stream 321 has asecond component 324 which passes along the cathode air passageway 106on the opposite side of the support member 20. As will be appreciated,the bifurcated cathode air stream 321 provides the necessary oxidant(oxygen, in the ambient air stream) to the cathode side 102 of themembrane electrode diffusion assembly 100. Yet further, the cathode airflow operates to remove less than a preponderance of the heat energygenerated by the membrane electrode diffusion assembly 100 while it isin operation. As will be recognized the cathode air flow is facilitatedby the respective cathode current collectors 60 which create in part,the cathode air passageway 106.

The anode heat sink air stream 322 is further bifurcated into a firstcomponent 325 and a second component 326, both of which individuallymove along the opposite sides 16 of the ion exchange membrane fuel cellmodule 10, and over each of the anode heat sinks 170. As the anode heatsink air stream components 325 and 326 move over the opposite anode heatsinks 170, the anode heat sink air stream operates to remove apreponderance of the heat energy generated by the ion exchange membranefuel cell module 10 during operation. Therefore, it will be recognizedthat the present invention provides an ion exchange fuel cell module 10which has a bifurcated air flow 320 which regulates the operationaltemperature of the ion exchange membrane fuel cell module by removingthe heat energy generated therefrom.

Referring now to FIG. 8, and as earlier discussed, the individual ionexchange membrane fuel cell modules 10 and the subrack 210 comprise, incombination, a fuel cell power system 5 which is coupled in fluidflowing relation relative to a source of a substantially pure or dilutefuel generally indicated by the numeral 341 and/or 342. The fuel gassupply may comprise a source of bottled and compressed fuel gasgenerally indicated by the numeral 341, or a fuel stream which isprovided by a chemical reactor, or reformer 342 which produces the fuelstream for use by the individual ion exchange fuel cell modules 10. Aconduit 343 couples either fuel gas supply 341 or 342 with therespective ion exchange fuel cell modules 10 and the associated subrack210. When a chemical reformer 342 is provided, the reformer wouldreceive a suitable hydrocarbon stream such as natural gas, propane,butane, and other fuel gases and would thereafter, through a chemicalreaction release a fuel stream which would then be delivered by way ofthe conduits 343.

The present fuel cell power system 5 may also include a fuel gasrecovery and recycling system (not shown) which would recover orrecapture unreacted fuel gas which has previously passed through theindividual ion exchange fuel cell modules 11. This system, in summary,would separate the unreacted fuel gas and would return the unreactedfuel gas back to the individual ion exchange fuel cell modules forfurther use. This recovery system would be coupled with the byproductremoval line 238.

Referring to FIGS. 11 and 12, switching circuitry 400 is provided foreach subrack or subsystem 210. As earlier discussed, each subrack orsubsystem includes a plurality of fuel cells or fuel cell modules 10.Each of these fuel cell modules 10 and subsystems 210 have acorresponding nominal voltage output. In the arrangement as seen in FIG.11, the nominal voltage outputs of each of the respective subsystems 210are substantially similar. For purposes of this application, asubstantially similar nominal voltage of the plurality of subsystems(each including a plurality of fuel cell modules 10) would be a voltagewhich is less than about 10% of optimal voltage of the other subsystems210. It should be understood that this switching circuitry also worksparticularly well with the fuel cell arrangement shown in U.S. Pat. No.6,030,718. This fuel cell arrangement includes subracks similar to thatdescribed in this application. The depicted DC-DC switching circuitry400 includes an input comprising input terminals 420, 422, circuitry408; and an output terminal 421. Input terminals 420, 422 are configuredto couple in parallel with the individual fuel cell sub-systems 210.Terminal 420 comprises a positive DC terminal and terminal 422 comprisesa negative DC or ground terminal. The terminals 420 are electricallyisolated from the several subsystems which are shown.

Switching circuitry 400 is configured to couple selected fuel cellsubsystems 210 to an energy storage device 412 (FIG. 11). The electricalenergy storage device 412 comprises one or more batteries, capacitors,super-capacitors, ultra-capacitors or a combination of one or morebatteries with one or more of the capacitor types described above. Moreparticularly, terminals 421 and 422 are respectively coupled in parallelto negative and positive terminals 424 and 425 of the energy storagedevice 412. In addition to performing a switching function, circuitry408, in one embodiment, is configured to convert direct currentelectrical energy having a variable voltage from one of the sub-system210, into direct current electrical energy having a substantiallyconstant voltage at the terminals 424 and 425. In FIGS. 11 and 12terminals 422; 424; and 423 are substantially at the same potential.

As shown in FIG. 11, the fuel cell power system 5 includes, in oneembodiment, a power conditioning device 426 having DC inputs 427 and 428coupled to the energy storage device 412 and electrical outputs 429 and430 selectively coupled to a load. The power conditioning device 426allows the fuel cell power system 5 to be used with, for example,household AC systems or other appliances.

As shown in FIG. 12, the depicted arrangement of circuitry 400 comprisesa switch 414. In one embodiment, switch 414 comprises a metal oxidesemiconductor field effect transistor (MOSFET). Switch 414 is configuredto selectively couple one fuel cell subrack or sub-system 210 withelectrical energy storage device 412. Multiple switching circuitry 400is provided to couple a plurality of fuel cell subracks with theelectrical energy storage device 412. More particularly, in oneembodiment, a circuit 400 is provided for each module 10 of each subrack210.

In the described embodiment, controller 250 is configured to monitor atleast one operational parameter of the fuel cell power system 5 and tocontrol switch 414 responsive to the monitoring. For example, controller250 is configured to monitor a voltage of electrical energy storagedevice 412. Responsive to the monitoring, controller 250 operates switch414 to selectively couple terminal 420 with node 416 for selected fuelcell subracks or sub-assemblies of fuel cell 218 to charge electricalenergy storage device 412. For example, if the voltage of the energystorage device 412 decreases, due to an increase in load, the controllermay bring one or more additional subracks on line and couple them to theelectrical energy storage device 412.

The energy storage device 412 has a nominal voltage substantiallysimilar to the optimal voltage of the fuel cell 218. In this regard, asubstantially similar voltage would be one which within less than about10% of optimal voltage of the fuel cell. Still further, in an exemplaryapplication, it is desired to provide a substantially constant directcurrent voltage of a predetermined amount between output terminals 421,and 423. Accordingly, it is desired to provide a direct current voltagepotential which is just slightly greater across terminals 416 and 422 toaccount for the voltage drop across a diode 418. In the embodiment shownin FIG. 11 the energy storage device 412 has a nominal voltage ofgreater than about 12 Volts DC, which is substantially identical to theD.C. output across terminals 421, and 423. In certain embodiments thediode 418 may be eliminated.

In one embodiment, the controller 250 controls the coupling of the fuelgas supply 341 and/or 342 to the individual subracks 210 which areselected to be coupled to the energy storage device 412. In thisarrangement fuel gas is supplied only to the subracks 210 that arecoupled to the energy storage device. A separate controlled fuel gassupply 341 or 342 may be provided for each subrack 210 in onealternative embodiment. In yet a further embodiment, a common gas supplyis coupled to all or multiple subracks 210 but supply to each subrack isindividually controllable, e.g., by an electronic valve controlled bythe controller 250. Moreover in one of the embodiments, when thecontroller 250 decides to bring an additional subrack 210 on-line forcoupling to the energy storage device 412, sufficient time is providedfor that subrack to come up to standard operating voltage before it iscoupled to the energy storage device 412. Thus, the fuel gas supply 341and/or 342 may be provided to the subrack 210 before that subrack iscoupled to the energy storage device 412. When the controller decides totake a subsystem or a subrack 210 off-line, it is decoupled from theenergy storage device 412 either simultaneously with decoupling of thegas supply from the subrack 210 or before or after decoupling of the gassupply.

Although a certain number of subsystems or subracks 210 are shown in thedrawings, and a certain number of fuel cell modules 10 are shown persubrack 210 in the drawing, it will be readily apparent that any desirednumber of subsystems or subracks and modules 11, or a portion thereof,could be employed in alternative embodiments.

Operation

The operation of the described embodiment of the present invention isbelieved to be readily apparent and is briefly summarized at this point.

An ion exchange membrane fuel cell power system 5 includes multiplemodules 10 each enclosing at least one membrane electrode diffusionassembly 100, and wherein at least one of the modules 10 can be easilyremoved from the ion exchange membrane fuel cell power system, by hand,while the remaining modules continue to operate.

FIGS. 13A, 13B and FIG. 13C together illustrate operation of thecontroller 250 and system 5 described above.

In step S1, the controller 250 is powered up (booted up).

In step S2 variables are initialized (e.g., thresholds or setpoints aredefined), and all switches 414 are opened to decouple the sub-systems210 from the energy storage device 412. After performing step S2, thecontroller 250 proceeds to step S3.

In step S3, the voltage of the charge storage device 412 is measured,and the status and availability of each sub-system 210 is checked. Afterperforming step S3, the controller 250 proceeds to step S4.

In step S4, a determination is made as to whether the measured voltageis less than a first threshold “SETPOINT 4” indicative of a very lowvoltage of energy storage device 412. If so, the controller proceeds tostep S5. If not, the controller proceeds to step S7.

In step S5, all the sub-systems 20 are de-coupled from the energystorage device 412. After performing step S5, the controller 250proceeds to step S6.

In step S6, the controller 250 asserts a signal to shut off the powerconditioning device 426 or otherwise disconnect the fuel cell powersystem 5 from the load.

In step S7, a determination is made as to whether the measured voltageis greater than or equal to a second threshold “SETPOINT 1” which isindicative of a high voltage at energy storage device 412. If so, thecontroller 250 proceeds to step S8. If not, the controller proceeds tostep S10 (FIG. 13B).

In step S8 (FIG. 13A), a determination is made as to whether allsub-systems 210 are de-coupled from the energy storage device 412. Ifso, the controller 250 proceeds to step S18 (FIG. 13C). If not, thecontroller 250 proceeds to step S9.

In step S9, all of the sub-systems are decoupled from the energy storagedevice 412 (since the voltage at the energy storage device is high).After performing step S9, the controller 250 proceeds to step S18 (FIG.13C).

In step S10 (FIG. 13B), a determination is made as to whether themeasured voltage is greater than or equal to a third threshold “SETPOINT2” which is indicative of a moderately high voltage at the energystorage device 412. If so, the controller 250 proceeds to step S11. Ifnot, the controller 250 proceeds to step S13.

In step S11, a determination is made as to whether all sub-systems 210are de-coupled from the energy storage device 412. If so, the controller250 proceeds to step S3 (since there are no sub-systems 210 toelectrically decouple from the energy storage device 412). If not, thecontroller 250 proceeds to step S12.

In step S12, one of the sub-systems 210 coupled to the energy storagedevice 412 is de-coupled from the energy storage device 412 (since thevoltage is moderately high, there are more sub-systems 210 coupled tothe energy storage device 412 than necessary, so one will be decoupled).The controller then proceeds to step S3.

In step S13, a determination is made as to whether the measured voltageis greater than or equal to a fourth threshold “SETPOINT 3” indicativeof a moderately low voltage. If so, the controller 250 proceeds to stepS14. If not, the controller proceeds to step S16.

In step S14, a determination is made as to whether all sub-systems 210are coupled to the energy storage device 412. If so, the controller 250proceeds to step S3 (since there are no additional sub-racks 210available to be coupled to the energy storage device 412). If not, thecontroller 250 proceeds to step S15.

In step S15, one of the sub-systems 210 that is de-coupled from theenergy storage device 412 is coupled to the energy storage device 412.After performing step S15, the controller 250 proceeds to step S3.

In step S16, indicative of a low voltage, a determination is made as towhether all sub-systems 210 are coupled to the energy storage device. Ifso, the controller 250 proceeds to step S3 (since there are noadditional sub-systems 210 that can; be coupled to the energy storagedevice 412 to raise the voltage). If not, the controller 250 proceeds tostep S17.

In step S17, all sub-systems 210 are coupled to the energy storagedevice 412. Following this step the controller proceeds to step S3.

Referring now to FIG. 13C in step S18 a determination is made regardingwhether an a power conditioner remote shut-off signal has been asserted.If so, the controller proceeds to step S19, if not the controller 250proceeds to step S3. In step S19 a determination is made whether a giventime delay is complete, if so, the controller proceeds to step S20, ifnot the controller proceeds to step S3. In step S20 the controllerclears the remote shut-off signal and then proceeds to step S3.

Thus, an appropriate number of sub-systems 210 are coupled to the energystorage device 412 depending on the voltage of the energy storage device412. Still further, the energy storage device 412 absorbs sudden spikesin the load without risk of damage to the sub-systems 210 and thuspermits the sub-systems 210 some time to come on-line.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A fuel cell power system comprising: a fuel cell which has an optimalvoltage; an energy storage device having a nominal voltage substantiallysimilar to the optimal voltage of the fuel cell; and an electricalswitch that, in operation, selectively electrically couples the fuelcell to the energy storage device to charge the energy storage device.2. A fuel cell power system as claimed in claim 1, and furthercomprising a plurality of additional fuel cells respectively havingoptimal voltages substantially similar to the optimal voltage of thefirst mentioned fuel cell.
 3. A fuel cell power system as claimed inclaim 1, wherein the fuel cell comprises multiple fuel cell subrackswhich selectively support respective fuel cell modules, and wherein therespective fuel cell modules can be operatively removed from theindividual subracks while the subracks remain operational.
 4. A fuelcell power system as claimed in claim 1, wherein the electrical switchselectively electrically couples the fuel cell to the electrical chargestorage device without any intermediate power conditioning or powerconversion.
 5. A fuel cell power system comprising: a plurality of fuelcells, the fuel cells respectively having substantially similar nominalvoltages; an energy storage device having a nominal voltagesubstantially similar to that of each of the fuel cells; and electricalswitching circuitry electrically coupled to the fuel cells and theenergy storage device, and wherein the electrical switching circuitry isconfigured to electrically couple a selectable number of the fuel cellsto the charge storage device to maintain the voltage of the chargestorage device above a predetermined voltage.
 6. A fuel cell powersystem as claimed in claim 5, wherein the fuel cells are defined by fuelcell stacks.
 7. A fuel cell power system as claimed in claim 5, whereinthe fuel cells are independently operable.
 8. A fuel cell power systemas claimed in claim 5, wherein the fuel cells are constructed andarranged so as to be able to be removed and replaced while the fuel cellpower system is in operation.
 9. A fuel cell power system comprising: afuel cell which has a nominal operating voltage; an energy storagedevice having a nominal voltage substantially similar to the nominaloperating voltage of the fuel cell; an electrical switch selectivelycoupling the fuel cell to the energy storage device; and a controllercoupled in voltage sensing relation relative to the fuel cell and theenergy storage device, and further coupled in controlling relationrelative to the electrical switch, the controller selectivelycontrolling the switch to selectively electrically couple the fuel cellto the energy storage device to maintain the voltage of the energystorage device above a predetermined threshold.
 10. A fuel cell powersystem as claimed in claim 9, wherein the energy storage devicecomprises a plurality of batteries.
 11. A fuel cell power system asclaimed in claim 9, wherein the energy storage device comprises acapacitor.
 12. A fuel cell power system as claimed in claim 9, whereinthe energy storage device comprises a battery and a capacitor.
 13. Afuel cell power system as claimed in claim 9, wherein the controllerelectrically couples the fuel cell to the energy storage device withoutany intermediate power conditioning or power conversion.
 14. A methodcomprising: providing a fuel cell having a nominal voltage; providing anenergy storage device having a nominal voltage which is substantiallysimilar to the nominal voltage of the fuel cell and electricallycoupling the energy storage device to a load; and selectivelyelectrically coupling the fuel cell to the energy storage device tosubstantially maintain the energy storage device above a predeterminedvoltage threshold.
 15. A method according to claim 14, and whereinproviding a fuel cell comprises electrically coupling a plurality ofselectively removable fuel cell subracks together.
 16. A methodaccording to claim 14, and further comprising: providing a controllercoupled in voltage sensing relation relative to the fuel cell and to theenergy storage device; monitoring the voltage of the energy storagedevice and the voltage of the fuel cell; and controlling the electricalcoupling of the fuel cell to the energy storage device with thecontroller responsive to the monitoring.
 17. A method according to claim16, and further comprising: providing a power conditioning device andelectrically coupling the power conditioning device to both the energystorage device and the load.
 18. A method according to claim 17, andfurther comprising: electrically coupling a sensor to the energy storagedevice to sense the voltage of the energy storage device and the voltageof the fuel cell, and electrically coupling the sensor to thecontroller.
 19. A method according to claim 18, and wherein providing afuel cell comprises: providing multiple independently operable fuelcells, and wherein the independently operable fuel cells may becomeinoperable without causing the remaining fuel cells to be renderedinoperable.
 20. A method according to claim 14, and wherein providing afuel cell comprises: providing a subrack for releasably supporting aplurality of ion exchange membrane fuel cell modules; and providing a DCbus which releasably electrically couples with the ion exchange membranefuel cell modules and electrically coupling the DC bus to the energystorage device.